Receiver efficiency of a solar power receiver greatly affects the optimum operating point of a power plant and reduced receiver efficiency causes a decrease in the optimum operating point of a power plant. Receiver efficiency is typically reduced due to reflection losses, convection losses and radiation losses from a solar power receiver.
Reflection losses occur if a surface at the receiver is incapable of absorbing or transmitting an incident solar ray. The relationship between absorption, transmittance and reflection can be illustrated by the equation 1=α+ρ+τ, where ρ is the reflectance, τ is the transmittance and α is the absorbance. Convection losses are experienced if a small air particle gains energy from a surface at higher temperature, and carries the energy away from the receiver.
Radiation losses are mostly influenced by the surface area from which a material radiates, the view factor which the material has with respect to another material of different surface temperature and the temperature itself of the emitting material.
Over the years, many solar power receivers have been made in a number of different configurations in order to minimize the losses mentioned above. Prior receivers that can generate hot pressurized air include tubular receivers and closed volumetric receivers. Tubular receivers are termed indirectly-irradiated receivers due to the fact that the working fluid is not directly exposed to the concentrated solar irradiation. On the contrary closed volumetric receivers are regarded as directly-irradiated receivers.
Tubular receivers generally consist of multiple high temperature resistant metal alloy boiler tubes through which a working fluid is passed such as compressed air, water/steam, carbon dioxide or any other suitable working fluid.
In first generation tubular receivers the tubes were placed externally along the periphery of a cylindrical tower. However, convection and radiation losses were too high.
Thus, in the next generation of tubular receivers, the absorber tubes were placed in a chamber. The energy is absorbed by the absorber tubes and passed through the tube wall to the working fluid that is contained within a closed circuit.
Closed volumetric receivers, on the other hand, make use of a pressurized quartz window through which solar irradiation passes and strikes a porous absorber medium inside a pressurized chamber. Pressurized air moves through the absorber medium and thus gains thermal energy while cooling down the absorber medium. As a consequence of the physical characteristics of suitable quartz glass, and the fact that it needs to be cooled during use, certain difficulties present themselves in the use of such an arrangement.
A slight deviation from a closed volumetric receiver is an open volumetric receiver. Open volumetric receivers are also directly irradiated receivers. In this instance ambient air, instead of pressurized air, is sucked through the absorber medium that is exposed to concentrated solar radiation. No quartz window is used and the disadvantages associated with a quartz window are avoided. However, a limitation of an open volumetric receiver is that it can only be used in a Rankine cycle for the production of electrical energy. Pressurized air cannot be contained and thus cannot be used as a working fluid.
There is a need for an improved central receiver for receiving reflected solar radiation from a heliostat field.